Many conventional analysis techniques measure the concentration of a chemical in a test specimen or sample, even where the specimen contains a complex mixture of chemicals. Such techniques include mass spectrophotometry, nuclear resonance, flame photometry, conductance and refractometry. While these techniques work, unfortunately, their accuracy is too often directly related to their cost. Further, many such techniques alter or destroy the specimen under test, and require relatively elaborate equipment.
More recently attempts have been made to determine various properties of materials, using sound, electromagnetic waves, or single pulses as the basis for analysis. In contrast to conventional chemical analysis, wave and pulse-based techniques can provide a non-invasive in vivo analysis.
For example, U.S. Pat. No. 4,679,426 (Jul. 1987), assigned to assignee herein, discloses a non-invasive in vivo technique for measuring concentration of chemicals, sodium chloride for example. Periodic electromagnetic waves having a repetition rate of about 10 MHz to 100 MHz were coupled to a subject's finger, and sodium or chloride ions within the finger apparently distorted these waves. This distortion in the composite waveform was received from the finger, using the same electrode-antenna pair used to couple the waves to the finger. The composite waveform distortion was then examined, and found to provide meaningful data as to chemical concentrations.
Glucose is an especially important chemical, a knowledge of whose absolute concentration level can be vital to diabetics. Several techniques for providing blood-sugar analysis are known, which permit subjects to determine their own glucose levels. Unfortunately many such techniques require invasive sampling of the subject.
One non-invasive technique for determining glucose levels in vivo was disclosed in U.S. Pat. No. 4,765,179 (Aug. 1988), also assigned to assignee herein. A periodic train of electromagnetic energy, preferably having a repetition rate of about 1 MHz to 1 GHz, was coupled to a subject's finger. The composite waveform distortion was then analyzed and found to provide meaningful analysis of glucose levels in the range of about 50 to 150 mg percent. However, beyond about 110 mg percent, it was desirable to fine-tune the electromagnetic energy to maintain measurement accuracy.
Understandably, glucose is a complex chemical. Monitoring the concentration of glucose in blood presents substantial challenges to discriminate against other materials in the blood that may mask or alter the analysis results.
What is needed is a non-invasive in vivo apparatus and method for determining a chemical level in a subject, including the chemical glucose. Preferably a lay person should be able to determine the chemical level, for example a diabetic determining his or her own glucose level. The present invention provides such apparatus and method.